Scattering, absorption and extinction of electromagnetic radiation passing through an interacting medium can be used to analyze material properties of the medium. The medium may force the electromagnetic radiation to deviate from its trajectory due to non-uniformities of the medium through which the radiation passes. As an example, a photodetector (e.g. a phototransistor or photodiode) can measure the light passing through a material. The pattern and intensity of the measured light provides information about the material and its properties.
Milk is a complex biological fluid with several constituents dispersed or dissolved in an aqueous medium. The composition varies between milk from animals of different species, between milk from animals of the same species, and between milk from the same animal depending on the season, feed and lactation stage. Milk may also include other minor constitutes like somatic cells, urea (non-protein nitrogen), microorganisms and residual compounds, including antibiotics, pesticides and heavy metals.
Fat in milk is present in the form of dispersed spherical globules. Depending on several intrinsic and extrinsic factors, it may or may not be in a crystalline or a semi-crystalline form. A layer of Milk Fat Globule Membrane (MFGM) emulsifies the globules in raw milk. It is a complex biological layer mainly composed of proteins and phospholipids. The diameter of the globules in raw milk varies with species and seasons. Its diameter is usually between 3-5 μm and up to 10 μm, and usually follows a lognormal distribution. In processed milk, the size is decreased to 0.7-2 μm by homogenization to increase globule surface area and promote emulsion stability.
Protein in milk is present as dispersed casein micelles and dissolved whey or serum. The former constitutes approximately 80% of the total milk protein. Casein micelles are stable spherical particles with mean diameter between 100-150 nm, and range between 50-500 nm. Milk is hence a polydisperse emulsion with dispersed particles (fat globules and casein micelles) having widely different particle size distributions. In homogenized milk, both casein and whey proteins are adsorbed to smaller fat globules as emulsifiers and form a layer usually thicker than the original MFGM layer.
The presence of multiple chemical constituents, in different physical forms, and varying concentrations significantly complicates interaction of electromagnetic radiation with milk. Most fundamental signatures due to atomic or molecular absorption from lipids, proteins and lactose lie in the mid-infrared region (2500-25000 nm) of the electromagnetic spectrum. Near-infrared region (780-2500 nm) also exhibits absorption signatures from milk constituents, however with overtones and band overlapping. Laboratory instruments for constituent analysis of milk typically use mid-infrared and near-infrared spectrums. Electromagnetic radiation is also scattered by milk due to the presence of particles of different sizes.
In the UV/Vis region (300-1100 nm), instead of specific bands and peaks of absorbance, a continuum is observed. Interaction is dominated by the scattering of light from fat globules and casein micelles that is strongly dependent on the ratio of particle diameter and incident wavelength. Equipment such as turbidimeters used for estimation of fat concentration measure the extinction of incident light due to scattering by fat globules.